A CCD typically comprises an array of light sensitive CCD elements. These elements are usually formed of a semiconductor material and generate charge when exposed to light of an appropriate frequency. In use, the CCD elements are exposed to incident light for an exposure time during which charge accumulates at the CCD elements. The charge accumulated at a CCD element depends upon the exposure time and intensity of light incident. Subsequent measurement of the charge generated at each CCD element corresponds to a measurement of the light incident on the CCD elements and allows an image to be recorded.
In order to measure the charge generated at each CCD element it is common to transport the charge away from the CCD elements to allow further exposures to be recorded while the recorded data is processed. This process of removing charge from the CCD and measuring it may be referred to as the transport and readout process. In order to transport the charge away it is common to draw the charge down from the array in columns. Thus charge from an element at the top of a column is drawn down through lower CCD elements to be read during the transport and readout process.
CCD devices are used in many imaging applications. In some, such as digital cameras, the CCD is provided behind a shutter. However, in others, such as luminescence and scintillation assay measurement systems, a shutter is not provided. However, the absence of a shutter can cause problems. In systems without a shutter, as the charge is drawn down through adjacent CCD elements during the transport and readout process, the CCD elements are still subject to illumination. This tends to increase the measured charge, particularly in cases where charge has been drawn through several CCD elements during the readout process. Further, this additional exposure time is not uniform for each charge. Charges corresponding to CCD elements at the top of a column are drawn down through several CCD elements during the transport and readout process whereas the charge associated with the lowest CCD element does not pass through any other CCD elements. As the illumination of CCD elements may not be uniform or constant, it is difficult to establish the effect on a charge of having passed through other CCD elements during the transport and readout process.
This problem may be illustrated, by way of example, with reference to FIGS. 1A and 1B. FIG. 1A shows a CCD 1 comprising one hundred light sensitive elements 10. The CCD is subject to illumination in 3 “spots” which fall on CCD elements marked A. During the transport and readout process charge is transported from the CCD in vertical columns, when in the configuration shown in FIG. 1B, leaving the CCD at the bottom of the CCD. The resulting image formed from the CCD data using standard processing is shown in FIG. 1B. As can be seen the three bright spots A have been recorded. However, there are also smears marked S which rather than showing unexposed areas show exposure. The smears S are due to charge accumulated as the charge from the CCD elements above the brightly exposed elements are drawn through the exposed elements during the transport and readout process. This problem of false image data caused by the transport and readout process is referred to as smearing.
As mentioned above, once the charge has been drawn from the CCD in columns a further exposure may begin. Alternatively, further current may be drawn down the columns, possibly more rapidly, in order to further “clear” the sensor prior to the next exposure. This process is known as clearing.
Another problem that may distort the recorded illumination data, is residual charge left during this clearing process. During clearing, current is drawn down the CCD in columns while the illumination still present. Because, the lower CCD elements receive charge accumulated in the CCD elements above them during the clearout process, the clearout process tends to leave more charge in the lower CCD elements than in the upper CCD elements. In some circumstances, this accumulation of charge may partially compensate for the smearing process described above. An uneven distribution of charge at the beginning of the exposure may also be a result of an incomplete or uneven clearout process after which an irregular concentration of charge is left at the CCD elements.
A known method of processing data from a CCD to reduce the effect of smearing on an image is linear offset approximation. If, for example in a scintillation assay, the CCD is arranged to measure light emitted from an array of wells, a lower portion of the CCD may be directed to view an area below the lowest well in the array and an upper portion of the CCD may be directed to view an area above the highest well in the array. Because no light is expected to be generated outside of the wells, it is expected that neither the upper nor lower portions of the CCD will be exposed. Due to the smearing effect described above, illumination data relating to the upper portion of the CCD may show some exposure due to smearing. In order to compensate for this artificial exposure, a measurement is taken of the exposure of the upper and lower portions of the CCD that were expected to be unexposed. The difference in exposure between the upper and lower portions of the CCD due to smearing is then measured and an offset is applied to the recorded data for each row of CCD elements equal to the measured exposure difference divided by the number of CCD elements between the upper and lower portions and multiplied by the number of rows of elements beneath that row.
The linear offset method assumes that the smearing is generated evenly throughout the CCD and works well when the illumination of the CCD is relatively uniform, for example when the wells of a scintillation assay measurement system are uniformly filled and each give out the same amount of light. However, when the exposure is non-uniform, such as that shown in FIG. 1A, the method tends to generate negative readings for those cells below the bright points of illumination and fails to fully compensate for the smearing above the bright points of illumination.
An object of the present invention is to provide a method of processing data from a CCD and a CCD imaging apparatus that tends to better compensate for the effect of the smearing described above.